U.S. Ser. No. 09/245,102, filed on Jan. 25, 1999 (the entire disclosure of which is hereby incorporated by reference) describes configurations that permit a single laser crystal to be driven by multiple pumping sources to obtain discrete, collimated outputs without substantial thermal crosstalk. FIG. 1 illustrates a generalized configuration as disclosed in this earlier-filed application. A recording medium 50, such as a lithographic plate blank or other graphic-arts construction, is affixed to a support during the imaging process. In the depicted implementation, that support is a cylinder 52 around which recording medium 50 is wrapped, and which rotates as indicated by the arrow. If desired, cylinder 52 may be straightforwardly incorporated into the design of a conventional lithographic press, serving as the plate cylinder of the press. Cylinder 52 is supported in a frame and rotated by a standard electric motor or other conventional means. The angular position of cylinder 52 is monitored by a shaft encoder associated with a detector 55. The optical components may be mounted in a writing head for movement on a lead screw and guide bar assembly that traverses recording medium 50 as cylinder 52 rotates. Axial movement of the writing head results from rotation of a stepper motor, which turns the lead screw and indexes the writing head after each pass over cylinder 52.
Imaging radiation, which strikes recording medium 50 so as to effect an imagewise scan, originates with a series of pumping laser diodes 60, four of which are representatively designated D.sub.1, D.sub.2, D.sub.3, D.sub.4. The optical components concentrate laser output onto recording medium 50 as small features, resulting in high effective power densities. A controller 65 operates a series of laser drivers collectively indicated at 67 to produce imaging bursts when the outputs of the lasers 60 are directed at appropriate points opposite recording medium 50.
Controller 65 receives data from two sources. The angular position of cylinder 52 with respect to the laser output is constantly monitored by detector 55, which provides signals indicative of that position to controller 65. In addition, an image data source (e.g., a computer) 70 also provides data signals to controller 65. The image data define points on recording medium 50 where image spots are to be written. Controller 65, therefore, correlates the instantaneous relative positions of the focused outputs of lasers 60 and recording medium 50 (as reported by detector 55) with the image data to actuate the appropriate laser drivers at the appropriate times during scan of recording medium 50. The driver and control circuitry required to implement this scheme is well-known in the scanner and plotter art.
The output of each of the lasers 60 is conducted, by means of an optical fiber 72.sub.1, 72.sub.2, 72.sub.3, 72.sub.4 to an alignment bench 75 that has a series of parallel grooves 77 for receiving the fibers. Bench 75, which may be fabricated from materials such as metal or silicon, is aligned with a laser crystal to direct the outputs of lasers 60 at appropriate points on the anterior face of laser crystal 80. To avoid substantial thermal crosstalk, the anterior face of the laser crystal (i.e., the side facing the pumping sources) may be provided with a series of parallel grooves and a pair of spaced-apart metal strips extending across the anterior face of the crystal perpendicular to the grooves. The strips and grooves serve to isolate thermomechanically the regions they define, and are aligned with the pumping sources such that the pumping-source outputs strike the anterior crystal face in the centers of the regions bounded by the strips and the grooves.
It is the emissions of crystal 80 that actually reach the recording medium 50. A first lenslet array 82 concentrates the outputs of lasers D.sub.1 -D.sub.4 onto crystal 80, and a second lenslet array 84 concentrates the outputs from crystal 80 onto a focusing lens 85. The latter lens, in turn, demagnifies the incident beams in order to concentrate them further and draw them closer together on the surface of recording medium 50. The relationship between the initial pitch or spacing P between beams from crystal 80 and their final spacing on recording medium 50 is given by P.sub.f =P/D, where P.sub.f is the final spacing and D is the demagnification ratio of lens 85. For example, the grooves 77 of bench 75 may be spaced 400 .mu.m apart, which also determines the pitch P. If the demagnification ratio of lens 85 is 4:1, then the spots will be spaced 100 .mu.m apart on the surface of recording medium 50.
Optimal performance requires precise alignment between fibers 72 and crystal 80. This can be difficult to achieve if, as is typically the case, bench 77 and crystal 80 are separate components mounted during assembly within a common fixture.